A conventional power amplifier (for example a class B, AB, F power amplifier) has a fixed RF load resistance and a fixed voltage supply. Class B or AB bias causes the output current to have a form close to that of a pulse train of half wave rectified sinusoid current pulses. The direct current (DC) and hence DC power is therefore largely proportional to the RF output current amplitude (and voltage). The output power, however, is proportional to the RF output current squared. The efficiency, i.e. output power divided by DC power, is therefore also proportional to the output amplitude. The average efficiency is consequently low when amplifying signals that on average have a low output amplitude (or power) compared to the maximum required output amplitude (or power).
One method for obtaining a high average efficiency from a power amplifier is called load modulation (LM), also known as dynamic load matching, for example as discussed in a paper by F. H. Raab entitled “High-Efficiency Linear Amplification by Dynamic Load Modulation,” 2003 IEEE MTT-S Digest. Dynamic load matching is a method that, by continuous re-matching of an output matching network, makes the apparent load resistance at the output of the power amplifier higher for low amplitudes. This means that for a specific output amplitude (below maximum), less RF current will be used than in the conventional constant load, constant supply voltage system described above. Such a system is shown in FIG. 1, whereby the power amplifier 1 receives an RF input signal 3, the output 5 of the power amplifier 1 being coupled to a tunable matching network 7 which under control of a control signal 9 dynamically matches the load to produce an RF output signal 11. Due to such re-matching of the load using the tuneable matching network 7, the RF voltage rises, possibly up to the limit set by the usually constant supply voltage. This is illustrated in FIGS. 2a and 2b, FIG. 2a showing the relationship between voltage and output amplitude, and FIG. 2b the relationship between current and output amplitude.
Operating a single radio frequency power amplifier at two widely separated frequency bands simultaneously can have the benefit of reducing the cost of a basestation or similar equipment, since the output power capability of the power amplifier could be shared between bands. This is sometimes called (power) pooling. This way, if all users are in one of the bands, all output power can be used for those users; if there are users in several bands, the output power capability can be divided between bands as appropriate. Multiband operation and power pooling do not generally cause any problems (above common engineering skills) in conventional power amplifiers.
However, it is difficult providing a load modulated power amplifier that can work with high average efficiency for simultaneous signals in widely separated frequency bands, since dynamic modulated matching networks are in practice quite narrowband in nature. The possibility of modulating to a wide range of apparent load impedances causes a narrow pass-band response in the matching network. The pass-band width of a matching network can generally be increased by using more matching stages and consequently more (in this case both tunable and fixed) components. This has the disadvantage of increasing the cost of such systems.
A further disadvantage of such systems is their inability to handle multiband signals with widely separated bands, due to the high speed at which the multiband signal varies. In particular, it is difficult for the tuneable reactive components in the matching network to be re-tuned at such high speed. This has potentially two bad consequences. First, a wideband matching network with multiple stages is electrically long which means that, at any point in time, it will contain up to several RF cycles worth of RF energy. The usual quasi-static conditions for passive matching then no longer apply, and speedy re-tuning causes parametric intermodulation and partially incoherent (time-mismatched) reflections. Second, re-tuning the reactive components (typically varactors) generally has a power cost proportional to the tuning speed (bandwidth) squared (i.e. proportional to both slew rate and repetition rate). As a result, for wide band separation the high tuning speed can reduce efficiency quite substantially.